Generally, a hybrid cell includes a photorefractive window and a layer of liquid crystal or polymeric material. In existing hybrid photorefractives, the evanescent space-charge field from one or more photorefractive windows induces a modulation of the refractive index in one or more adjacent layers of liquid crystal or polymeric material. As a result, the omnipresent evanescent photorefractive space-charge field increases the overall device index modulation by inducing an additional index modulation in adjacent high birefringence material(s) which may or may not themselves be inherently photorefractive.
More specifically, on exposure to optical intensity gradients (such as the interference between two mutually coherent laser beams) photogenerated charges diffuse to create a modulated space-charge field within the inorganic windows of the hybrid cell. Penetration of the surface space-charge field into the liquid crystal layer modulates the director alignment of the liquid crystal molecules, creating diffractive beam coupling within the liquid crystal layer. In practice, this inherent simplicity belies a more complex nature. The beam coupling is unidirectional, leading to strong power coupling from one beam to another irrespective of the relative intensities of the beams. The origin of the large unidirectional gain in the liquid crystal layer arises from a combination of local surface induced pre-tilt of the liquid crystal molecules together with splay-induced flexopolarization of the nematic liquid crystal. A pre-tilt of the liquid crystal molecular surface alignment permits spatial frequency matching of the optical interference pattern and the resulting index grating, while splay induced flexopolarization enables the otherwise nematic material to become sensitive to the sign of the space-charge field.
Current hybrid cells have been successful, but the potential benefits of the hybrid photorefractive technology have been limited by the relatively small magnitude of the available evanescent space-charge field. Great improvements to the hybrid device performance are possible if either the space-charge field can be increased significantly, or if the sensitivity of the adjacent liquid crystal or polymer layer(s) to the influence of the electric field can be improved. One example of such hybrid device would include a liquid crystal layer with a 45 degree pretilt, with preferably a low anchoring energy. In devices where charge migration is dominated by diffusion and also in situations where the application of external electric fields is undesirable, increasing the sensitivity of the liquid crystal or polymer layer(s) to the space charge field is the most practical method for improving device performance. As such, there is a need for improved photorefractive hybrid cells.